/Engine/Source/Runtime/Engine/Classes/GameFramework/InputDeviceLibrary.h
Returns the 'Primary' Platform user for this platform. This typically has an internal ID of '0' and is used as the default platform user to map devices such as the keyboard and mouse that don't get assigned unique ID's from their owning platform code.
static [FPlatformUserId](API\Runtime\Core\Misc\FPlatformUserId) GetPrimaryPlatformUser()